Self-Assembled Superlattice Nanowires

Superlattices offer quantum confinement that gives rise to several useful quantum effects. Nanowire superlattices, wherein each layer is confined from all three dimensions, are essentially an ordered series of quantum dots that could serve as a building block in several quantum electronic applications, such as lasing and thermoelectric filtering. Attempts to grow nanowire superlattices involved meticulous control of growth condition that resulted in limited success and applicability.

In this work, we employ a naturally occurring self assembly mechanism that results in the formation of fast, accurate, periodic phase separation in semiconductor ternary materials. This effect has been observed in several oxides of Zn and another metal, where the two oxides have limited or no mutual miscibility. Here, we present results from a study of the In-Zn-O system. The stable polytypes of ZnO and InO are very different in their crystalline structure. At the typically very high growth temperatures used in CVD growth of nanowires, their ternary alloy undergoes a periodic phase separation into ZnO:In and In2O3. Currently, there is still no model to explain the phenomenon.

We report observations relating to the growth of In-Zn-O superlattice nanowires that may lead to better understanding of this self assembly. One is the relation between the superlattice axis and the crystal axes. We observed phase separation only along the c-axis of ZnO. When wires are grown in the c-direction, the superlattice is observed along the growth axis, while in nano wires grown in the a-direction the superlattice is perpendicular to the growth axis. Another observation is the relation between the nanowire diameter and the length of the superlattice period. TEM reveals a striking resemblance of the superlattice to another unexplained phenomenon known as periodic twinning, suggesting that the superlattice may be a special case of periodic twinning promoted by the presence of another metal.